The Es-9 will take up a lot of space in the rackbrute. I have a es8 + es6 which will give you 10 in which is a lot for a modular rig and also 8 out, This combo will give you an additional 4 hp back. Based on your Wishlist- you will be ok with an es8 to run your audio into Ableton on 4 separate channels.
Even if you want the keep your modular separate whilst you go through the learning curve, Having the e8 is extremely useful as a learning tool as you can use two of your outs with the scope in VCV Rack to view your signals. One equivalent of this functionality in eurorack is the mordax data which is big and will cost around £400